Bandsaw blades for milling timber

ABSTRACT

A bandsaw blade for use in milling timber having in-line swaged teeth, wherein the cutting tip of each tooth, in front profile, is triangular in shape with a central apical point (30) and two side points (32, 34). Preferably, the apical angle (θ) is from 60° to 140°, with the base angles (γ) of the tip being equal to one another and lying from 20° to 60°, thus providing an elongate pentagon shape in front profile. Also, a dark blue temper band (50) may be curved so that its top is spaced below the apical point (30) by a distance of 0.5 to 1.3 times, preferably 0.6 to 1.2 times, the blade thickness.

TECHNICAL FIELD

This invention relates to bandsaw blades for milling timber and, moreparticularly, to tooth forms for use in such blades. Its objective is toallow the production of bandsaw blades which have a longer effectivecutting time, improved production rates, lower power consumption andreduced kerf loss.

BACKGROUND ART

Though diminishing timber resources and increasing energy costs havecreated a strong demand for more efficient timber milling techniques,little attention has been paid to the improvement of tooth design. Thisis because saw tooth design is a well-worked art with a soundtheoretical and empirical base which has determined the tooth shapesused on large bandsaws for many years. A steady increase in bandsawperformance has been achieved over the past thirty years by the use ofimproved alloy steel for blades, particularly through the introductionof steels and equipment which allows saw teeth to be sharpened andlocally hardened at the mill. The tough alloys used reduce tip breakageand, when hardened, are resistant to abrasive wear, which isparticularly important when sawing highly siliceous timber. Basic sawtooth shape, however, has remained essentially unchanged during thisperiod.

DISCLOSURE OF INVENTION

In such a well-worked art and in such an economic climate, it would bemost surprising to find that productivity improvements of up to 30%could be obtained using existing saw blade alloys and hardeningtechniques by changing tooth profile and by carefully controlling tiphardness, yet the present invention demonstrates such improvements arerealistic. Instead of the conventional trapezoidal, or flat-top, toothshape employed for in-line and swaged bandsaws, the teeth formed inaccordance with the present invention each have a cutting tip which, infront profile, is triangular in shape, with, for instance, an elongatepentagonal or pointed-top, tooth shape, with a central apical point andtwo side points. Preferably, the apical angle (θ), namely, the includedangle of the central point is, from 60° to 140°, while the side or baseangles (γ) of the triangular tip are from 20° to 60°.

It is also preferable, but not essential, for the tooth hook angle, inside profile, to be significantly increased over that which isconventional. Thus, hook angles as large as 35° are now employed cuttingvery dense and silicious timbers, in which the conventional angles of upto 25° only were traditionally used. Similarly, the sharpness angle ispreferably reduced to improve penetration; angles of as little as 35°may be used with softwoods, though angles from 40° to 45° will be moreusual. Clearance angles of from 15° to 20° are thus contemplated.

The pointed-top tooth form allows the overall width of the tooth to besignificantly reduced without increasing lateral vibration when in use,so that valuable reductions in kerf loss can be obtained. For optimumperformance, it is recommended that teeth formed in accordance with thepresent invention should be high-frequency induction hardened to give atip hardness of between 900 and 920 Diamond Pyramid hardness number(DPN). A convenient way, according to the present invention, ofobtaining this hardness at the mill without using elaborate anddestructive hardness measurements, is to employ standard hardening gradeblade steels and to adjust the high-frequency coil inductor until a darkblue temper band is produced on the face of the tooth such that its toplies between 0.5 and 1.3 times the blade thickness below the tooth apex.Forced cooling by way of an air flow is preferably used to ensure thatall teeth are treated uniformly.

To further portray the nature of the present invention, a detailedcomparison between the performance of a bandsaw with conventional toothform and one with the tooth form of the present invention will bedescribed with reference to the accompanying drawings. First, however, abrief review of the theoretical considerations underlying the radicalnew tooth form will be provided, also with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

In the drawings:

FIG. 1A illustrates the front profile of a conventional bandsaw toothused for milling timber;

FIG. 1B illustrates the side profile of the bandsaw tooth of FIG. 1A;

FIG. 2A depicts the front profile of a bandsaw tooth formed inaccordance with the present invention, including a comparison with thefront profile of a conventional bandsaw tooth;

FIG. 2B depicts the side profile of the bandsaw tooth of FIG. 2A,including a comparison with the side profile of a conventional bandsawtooth;

FIG. 3A is a perspective view of a bandsaw tooth formed in accordancewith the present invention;

FIG. 3B is a second perspective view of the bandsaw tooth shown in FIG.3A;

FIG. 3C is a front profile of the bandsaw tooth shown in FIG. 3A,including a comparison with the front profile of a conventional bandsawtooth; and

FIG. 4 is a graphical comparison of the performance of three saw bladetypes; the upper part plots the percentage of optimally dimensionalchips produced against time, while the lower depicts saw-log inputagainst time.

MODES FOR CARRYING OUT THE INVENTION

A great many factors must be considered in providing any theoreticalanalysis of the cutting action of a saw tooth that takes into accountthe forces acting on the tooth as it is driven through a non-homogenoussubstance such as wood, as well as the mechanisms of chip production andtooth wear. Various authorities in the art have propounded over twentyfundamental and largely independent equations in conducting variouselements of this analysis. An excellent review of the art may be foundin the text "Wood Machining Processes" by P. Koch, Ronald Press, NY(1964). Unfortunately, little of the analysis can be applied directly tothe prescription of tooth form, this being left to empirical criteriaderived from experience; and this is particularly so in regard toeffective tooth life or the mean time between sharpenings. A "perfectlysharp" tooth is of little value if it has a rapid rate of wear. It isthus characteristic of the industry that most mills employ their own"saw doctor" who has his own recipe for tooth profiling whenresharpening.

Sawmillers appreciate that progressive dulling of the tooth cuttingpoint or edge adversely affects chip formation. Theoretical studiesconfirm this by pointing to the way in which tool forces causeincreasing compression and shear failure in the wood ahead of the toothedge as the tooth becomes blunter. They point to the existence of acritical sharpness angle which, if exceeded through tooth edge wear orbreakage, will have an adverse effect upon chip characteristics. Theinventor has demonstrated the existence of an optimal chip for a giventimber feed rate and blade type, it being shown that, when a highproportion of chips produced are of the optimum size, the blade iscutting efficiently. As the teeth wear, the percentage of thesewell-formed optimum chips decreases and the proportion of compressed andfragmented chips of smaller size increases. These studies were reportedin a paper entitled "Dimensional Saw-Chip Formation; Non-DestructiveIndication of Blade Behaviour" published in Holz als Rohund Werkstoff,35 (1977) pp 307-310. It was also demonstrated that the loss of metalfrom a saw during a milling operation was closely correlated to theblade condition as indicated by the proportion of optimum chips in thesawdust produced at any given time. This work was published in the aboveJournal in 1979 (No. 37, pp 353-358) in an article entitled"Non-Destructive Method for Continuous Sawtooth Wear Assessment".

The last-mentioned paper also demonstrated that there are two distinctphases of tooth metal loss; in the first, which occurs in the first 15minutes of cutting, metal is rapidly lost by brittle fracture as thesharp cutting edges come into contact with hard and abrasive portions ofthe timber; in the second, a succeeding and much longer period, metalloss is due to fatigue and occurs at a decreasing rate as the cuttingedge becomes rounded. Physical examination of a saw confirms thesefindings in a qualitive way and indicates that much of the metal loss inthe first phase is due to the side corners of the teeth being chippedaway. These studies were carried out upon saws used in general practicewith conventionally shaped teeth and, being prepared for hardwood, hadthe following characteristics:

Manufacture: Uddeholm,

Alloy Type: High-nickel; type ASSAB 5445,

Blade Thickness (t): 2.41 mm,

Blade Width: 350 mm,

Number of Teeth: 264,

Tooth pitch (p): 57 mm,

Gullet depth (h_(t)) 23 mm,

Hook angle (a): 25.0°,

Sharpness angle (B): 44.0°,

Clearance angle (γ): 21.0°,

Side swage (f_(s)): 0.8 mm,

Kerf width (k): 4.1 mm,

Tip hardness: 860 DPN.

FIG. 1 illustrates the tooth form of such a blade and identifies thevarious angles and dimensions by the letters given in brackets above.View A of FIG. 1 depicts the front profile of a tooth, while View Bshows the side profile. As clearly shown by view A, the swaged portionof the conventional tooth has a trapezoidal shape in front profile, thetrapezoid being bounded by the two cutting tips 10 and 12 and the twoswage shoulders 14 and 16. The cutting edge 18 of the tooth is the linewhich joins corners 10 and 12.

It can be concluded from the analysis outlined above that the corners(references 10, 12, FIG. 1A) of the conventional tooth are toovulnerable to overload and brittle fracture, but the only way to reducethis in such a tooth form, is to increase the sharpness angle (B); thatis, make the tooth blunter. In fact, the standard tooth shape indicatedin FIG. 1 represents an optimum balance between tip breakage andsharpness angle which has been struck by many years experience insawmilling throughout the world.

According to the present invention, however, it is possible to bothprovide better support for the tooth side corners 10, 12 and to reducethe sharpness angle (B) at the same time. This is achieved by forming athird, central or apical, tooth point (reference 30 in FIG. 2A), so thatthe tooth tip has a triangular shape, thereby allowing significantlylarger side corner angles and therefore better supported corners(reference 32, FIG. 2), which are less vulnerable to overload andbrittle fracture. Because the cutting force is now spread over threeshear points instead of two and because the cutting edge has now beensignificantly increased in length, the stress to which it is subject isvery greatly reduced, allowing the tooth hook angle to be significantlyincreased and the sharpness angle correspondingly reduced so that timberpenetration is greatly improved. In short, the saw cuts much better.

FIG. 2 contrasts the tooth form of the present invention (shown in solidlines) with the conventional (shown in dotted lines). It will be seenthat the swaged portion of the tooth has an elongated pentagonal shape(bounded by points 30, 34, 14, 16 and 32) instead of the conventionaltrapezoidal shape. The two new side cutting edges 36 and 38 which flankthe apical tooth point 30, each must have a suitable following bevelclearance to ensure proper cutting. This is shown by the perspectiveviews A and B of the new tooth form illustrated in FIG. 3, the bevelledclearance faces being indicated by 40 and 42 respectively. As shown byFIG. 2A, the angle of the side corners 32 and 34 is measured as the baseangle of the isosceles triangle 30, 32, 34 and is indicated by angle(γ).

Examination of FIG. 2A, reveals another significant advantage of the newtooth form; namely, the reduced kerf (K), which comes about by thebevelling of the tooth top so that the side corners are positioned loweron the tapering swaged portion of the tooth. Reduction of the effectivetooth width results in less kerf loss but a greater saving is possiblein this regard than is indicated by the simple geometry of thetriangular tooth tip. This is because, first, the tooth has been madestiffer by effectively lowering the cutting edge without increasing thegullet depth (h_(t)) and, second, the sloping sides 36, 38 of thetriangular tip tend to stabilise the tooth against lateral vibrationduring cutting, both factors thereby allowing further reduction in thethickness of the swage to give the same effective tooth stiffness andstrength as a conventional blade. A total reduction of overall toothwidth of up to 15% over conventional blades is possible, representing amost significant saving in timber.

The range of geometrical forms envisaged by the present invention hasalready been indicated above, but one specific example will now bedescribed so that comparative performance data can be given. Threebandsaw blade types are involved in this comparison:

(i) An unhardened conventional blade (C) having the tooth form listedabove and the blade hardness of 860-880 DPN;

(ii) a blade (M) having the new pentagonal tooth shape but otherwise notoptimised; and

(iii) a blade (E) having the pentagonal shape and the optimised toothform as indicated above.

                  TABLE 1                                                         ______________________________________                                        Parameters and Tooth Profiles                                                 for the C, M and E Blades.                                                              Sawtooth profiles                                                                           Partly    Fully                                                   Conventional                                                                              modified  modified                                    Parameters  (C)         (M)       (E)                                         ______________________________________                                        Pitch (p)       57 mm       57 mm   57 mm                                     Hook angle (α)                                                                          25.0°                                                                              30.0°                                                                          30.0°                              Sharpness angle 44.0°                                                                              40.7°                                                                          42.5°                              (β)                                                                      Clearance angle 21.0°                                                                              19.3°                                                                          17.5°                              (Υ)                                                                   Gullet depth    23 mm       23 mm   21 mm                                     (h.sub.t)                                                                     Gullet area (S) standard    standard                                                                              reduced                                   Gullet profile  not well    not well                                                                              fully rounded                             (S.sub.p)       rounded     rounded with base of                                                                  tooth                                                                         straightened                              Side swage (f)  0.75-       0.75-   0.60-                                                     0.80 mm     0.80 mm 0.65 mm                                   Slope angle (ω)                                                                         does not exist:                                                                           48.0°                                                                          37.0°                                              conventional                                                  Apic angle (θ)                                                                          flat surface                                                                              84.0°                                                                          106.0°                             Hardness        not hardened                                                                              860-901 915                                       (DPN)                                                                         ______________________________________                                    

These blades were used at the same mill on a stock of hardwood(tristania conferta) logs selected to be as uniform as possible. Theoverall results of these trials are indicated in Table 2 below and amplyconfirm the superiority of the new tooth form. However, as discussed inthe first of the two papers cited above, a more telling way to depictblade performance during use is on a graph which plots optimal chip sizeagainst duration of cut (or log input), it being shown that the bladeshould be taken off for resharpening when the proportion of optimum sizechips falls to <50%. FIG. 4 provides such a diagram for the averages ofthree blade types and the comparative trials described. From this, itwill be seen that the duration of Phase 1 (or optimal cutting) forblades formed in accordance with the present invention is not only muchlonger than for a comparable blade (M), but the rate of degradation ofthe blade during Phase II cutting is significantly less. Phase IIterminates at the ˜50% optimal chip level and leads to Phase III, inwhich damaging and counterproductive cutting occurs. The averageeffective cutting time per blade listed in Table 2 is the cutting timebefore sharpening. Although it approximates the end of Phase II cutting,it was in fact determined by the mill operator's decision to remove theblade for resharpening. The surprising degree of improvement is clearfrom Table 2, the graph of FIG. 4 and the log input diagram forming partof FIG. 4.

                                      TABLE 2                                     __________________________________________________________________________    Feed Speed Data for the C, M and E Blades                                         Average                                                                            Average       Time per                                                                            Mean feed                                                                           Average                                        log  depth                                                                              Average effective                                                                      average                                                                             speed per                                                                           feed speed                                 Blade                                                                             length                                                                             of cut                                                                             cutting time per                                                                       log length                                                                          blade per test                                   code                                                                              (m)  (m)  test (min)                                                                             (sec) (m/sec)                                                                             (m/min)                                    __________________________________________________________________________    C   4.37 0.38  48      8.3   0.527 31.5                                       M   4.02 0.42 114      7.6   0.539 32.3                                       E   4.31 0.44 130      6.4   0.673 40.4                                       __________________________________________________________________________

As indicated at the outset, optimal hardening of the teeth formed inaccordance with the present invention is critical, as it would be forany tooth shape which was to be hardened. It has been found that alloysteels commonly employed in the production of large bandsaw blades forsawmilling are reasonably uniform in character and that reliance may beplaced upon tooth face colour after these were hardened, as a preciseindicator of optimal hardening. More specifically, after high frequencyhardening, the face of a tooth formed in accordance with the presentinvention should have a dark blue upwardly-curved temper band visible,the topmost central edge of which is spaced between 0.6 and 1.2 timesthe blade thickness from the apical tip, and preferably between 0.2 and1.2 times the blade thickness below the swaged side tips of the tooth.The position of the dark blue temper band is indicated in FIG. 3C, byreference 50. The precise location of this band on (E) profile sawsmanufactured from Uddeholm steel, which were used in the trialssummarised in Table 2 is 1.0 times the blade thickness for the distanceA-D (FIG. 3C), and 0.9 times the blade thickness below the swaged sidetips of the tooth. It should be noted also, that in the comparativetrials described above, blades (M) and (E) were both high-frequencyhardened using the same induction hardening machine.

Having described one comparative trial on hardwood using a specifictooth profile formed in accordance with the present invention, thoseskilled in the art will appreciate that some adjustment in the toothprofile is always needed to suit the particular timber species beingcut. Therefore, any triangular tooth form within a fairly wide range canbe employed without departing from the scope of the present invention.For example, when cutting P, radiata, which is soft and has littlesilica inclusions, more sharply pointed teeth can be employed. An apicalangle as little as 80° may be suitable (giving base angles of 50° each)and a hook angle of as much as 35° may be used in conjunction with asharpness angle of as little as 35° and a clearance angle of 20°.However, teeth of such extreme form need careful hardening, asdescribed, even when used on softwoods, where the feed rates well inexcess of those feasible with conventional blades may be expected. Atthe other end of the spectrum it may be desirable to use apical anglesof as much as 103° (giving base angles of as little as 30°) and a hookangle of as little as 26° and a sharpness angle of as much as 45° whereextremely difficult hardwood species or other abrasive materials are tobe processed. More usual will be the case where the range of mediumdensity hardwoods are being cut, and where hardening can be carried outas prescribed. In such cases, the tooth form provided in the specificexample described will provide an excellent form from which to start.

It should be noted also that in future, the application of the new sawtooth profile might be extended toward processing other types ofmaterial than timber, including metal and plastics.

I claim:
 1. A bandsaw blade for use in the milling of timber and whichhas in-line swaged teeth; the swaged portion of each tooth being shapedas an elongate pentagon which blends uniformly into adjacent unswagedregions of the tooth, the swaged portion of each tooth having a frontface which is substantially planar, a hook angle of 25° to 35°, asharpness angle of 40° to 45°, a clearance angle of 15° to 20° and acutting tip which, in front profile, is triangular in shape, thetriangular cutting tip having a central apical angle falling within therange of 60° to 140° and base angles falling within the range of 20° to60°, and is hardened to a hardness level falling within the range of 900to 920 DPN.
 2. The bandsaw blade as claimed in claim 1 wherein eachtooth is hardened in a manner such that a dark blue temper band ispresent with an uppermost portion of the band being spaced below theapical point by a distance corresponding to 0.5 to 1.3 times the bladethickness.
 3. The bandsaw blade as claimed in claim 1 wherein each toothis hardened to such an extent that a dark blue temper band is presentwith an uppermost portion of the band being spaced below the apicalpoint of the tooth by a distance corresponding to 0.6 to 1.2 times theblade thickness and wherein the temper band is curved such that it isspaced from side points of the tooth by a distance corresponding to 0.2to 1.2 times the blade thickness.